Quality parameters of photomask-forming silica glass substrates include the size and density of defects, flatness, surface roughness, the opto-chemical stability of material, and the chemical stability of surface. More strict requirements are imposed on these parameters as the design rule becomes ultra-fine. In connection with the flatness of photomask-forming silica glass substrates, it is required to provide a glass substrate having not only a flatness in the acceptable range, but also a sufficient shape or topography to ensure that an exposure surface of a photomask prepared therefrom is flat during exposure. If the exposure surface is not flat during exposure, the exposure light suffers off-focusing on a silicon wafer to exacerbate the pattern uniformity, failing to form a precise micropattern.
The exposure surface of the photomask (surface subject to light exposure during an exposure step) is required to have a flatness of up to 2.2 nm per square centimeter of the exposure surface area, or up to 0.5 μm for the 6025 size (152 mm by 152 mm by 6.35 mm) photomask. Typically, the photomask is prepared by forming a light-shielding film on a silica glass substrate and patterning the film. On use, the photomask is most often set horizontally during the exposure step by holding the peripheral surface of the photomask by means of a suction grip or the like. Then, the shape of the photomask during the exposure step is governed by not only the flatness of the substrate, but also the resultant force of the film stress, gravity warp and holding force. These factors differ depending on the quality and thickness of the light-shielding film, the patterning mode and the exposure apparatus. The flatness of the exposure surface is largely affected by the flatness of the hold surface of the photomask substrate during the exposure step although this is not always the case. It is then required that the portion of the silica glass substrate corresponding to the hold surface of the photomask have a higher degree of flatness. Specifically, the portion of the silica glass substrate corresponding to the hold surface of the photomask should preferably have a flatness of up to 1.3 nm/cm2. Such local leveling is, in fact, impossible with current leveling methods based on the traditional polishing technology.
If the entire surface of a substrate is made flat, its portion corresponding to the hold surface is also flat. In the existing precision polishing methods such as rotary double-sided lapping on a batchwise basis and rotary single-side lapping on a single-wafer basis, soft abrasive cloths are used in the polishing of substrates so as to incur no or few defects. It is, in fact, impossible to level out non-uniformities over the entire substrate surface including its periphery.
When the factors associated with film stress and gravity warp must be taken into account, a technique capable of establishing a more complex arbitrary shape is needed, and the traditional precision polishing technology comes to the limit of its abilities.